Stretched films obtained by stretching resin are used as optical films that achieve various optical functions in various types of display devices, utilizing their optical anisotropy. For example, it is known that in a liquid crystal display device, the stretched film is used as an optical compensation film such as for optical compensation such as coloring prevention and wide viewing angle, and that the stretched film is adhered to a polarizer, and thus the stretched film is used as a retardation film which also functions as a polarizing plate protective film.
On the other hand, in recent years, as a new type of display device, much attention has been focused on self-luminous display devices such as an organic EL (electroluminescence) display device. In the self-luminous display device, there is a room for reduction of power consumption as compared with a liquid crystal display device in which a backlight is constantly on. Furthermore, in the self-luminous display device, such as the organic EL display device, in which light sources corresponding to individual colors are individually turned on, since it is not necessary to provide a color filter that is a factor for reducing a contrast, it is possible to further increase the contrast.
However, since in the organic EL display device, in order to increase the efficiency of taking out light, a reflective member such as an aluminum plate is provided on the back surface side of a display, external light entering the display is reflected off the reflective member, with the result that an image contrast is disadvantageously reduced.
Hence, it is known that in order to enhance a light and dark contrast through external light reflection prevention, the stretched film is adhered to a polarizer to form a circular polarizing plate, and the circular polarizing plate is used on the surface side of the display. Here, the circular polarizing plate is formed by adhering the polarizer to the stretched film such that the in-plane slow axis of the stretched film is inclined at a desired angle with respect to the transmission axis of the polarizer.
However, a general polarizer (polarizing film) is obtained by being stretched at high magnification in a transport direction, and its transmission axis is aligned with a width direction. Thus, a conventional retardation film is manufactured by vertical stretching or lateral stretching and, in principle, the in-plane slow axis is pointed in a direction at 0° or 90° with respect to the longitudinal direction of the film. Hence, in order to incline, at a desired angle, the transmission axis of the polarizer and the slow axis of the stretched film as described above, there is no choice but to adopt a batch method of cutting a long polarizing film and/or a stretched film into film pieces at a particular angle and of adhering the film pieces one by one. This disadvantageously results in poor productivity and low product yield due to attachment of shavings or the like.
In order to overcome this problem, there are proposed various methods for manufacturing a long retardation film that allows a film to be stretched in a direction at a desired angle (in an oblique direction) with respect to the longitudinal direction and that allows the direction of the slow axis to be controlled to be an arbitrary direction neither at 0° nor at 90° with respect to the longitudinal direction of the film. For example, in the manufacturing method disclosed in patent document 1, a resin film is fed out from a direction different from the winding direction of the film after the stretching of the resin film, and is transported with both end portions of the resin film held with a pair of holding members. The transport direction of the resin film is changed partway and thus the resin film is stretched in an oblique direction. In this way, a long stretched film is manufactured that has a slow axis at a desired angle more than 0° but less than 90° with respect to the longitudinal direction.
By use of such a stretched film having the slow axis inclined with respect to the longitudinal direction, it is possible to manufacture a circular polarizing plate by adhering a long polarizing film and a stretched film on a roll to roll basis instead of adhereing by a conventional batch method. Consequently, the productivity of the circular polarizing plate is dramatically enhanced, and its yield is significantly improved.
However, when the film obliquely stretched as described above is applied to a circular polarizing plate for external light reflection prevention in an image display device with a significantly high contrast such as a large organic EL television (OLED (organic light-emitting diode)-TV), it is found that when a black display is produced, the level of light leakage of external light reflected light in the circular polarizing plate varies depending on the position of a display screen, that is, so-called variations in the amount of reflected light occur. This is probably because the oblique stretching causes variations in orientation angle in the width direction of the film, and consequently, it is impossible to obtain satisfactory optical properties over the entire film. The orientation angle refers to an angle formed between the direction of the slow axis of molecules of the film and the width direction of the film.
In this point, in the manufacturing method disclosed in patent document 1, a tension in a direction in which the stretched film is drawn is controlled to fall within a given range, and thus variations in the thickness of the film in the width direction are reduced, with the result that the film with little variation in the optical properties attempts to be manufactured. However, in the manufacturing method disclosed in patent document 1, since the tension in the drawing direction of the film is drawn is constant in the width direction of the film, it is impossible to reduce variations in the orientation angle in the width direction, with the result that it is impossible to obtain satisfactory optical properties over the entire film.
Hence, in order to reduce variations in the amount of reflected light when a black display is produced as described above, it is necessary that the orientation angle of the stretched film can be adjusted in the width direction and that variations in the orientation angle in the width direction caused by stretching can be reduced.